The present invention relates to a copier, printer, facsimile transceiver or similar photographic image forming apparatus and a charging device thereof and, more particularly, to an image forming apparatus of the type using a charge roller for uniformly charging the surface of a photoconductive element, or image carrier, during a sequence of image forming steps, and a charging device thereof.
It has been customary with an image forming apparatus of the type described to use a corona discharger as charging means for uniformly charging the surface of a photoconductive element. A corona discharger effectively charges the surface of a photoconductive element uniformly to a predetermined potential. However, the problem is that a corona discharger needs a high tension power source and generates ozone during discharge. Ozone generated in a great amount would not only pollute the environment but also aggravate the deterioration of a charging member as well as the photoconductive element.
In light of this, there has been proposed a charging device using a charge roller in place of the corona discharger. This type of charging device has a charge roller held in contact with and driven by a photoconductive drum. The charge roller has a metallic core. As a voltage is applied from a power source to the core of the charge roller, the roller charges the surface of the drum. With the charge roller, it is possible to lower the required voltage of the power source and to reduce the amount of ozone ascribable to charging. In addition, the charge roller prevents dust particles from electrostatically depositing on a corona wire and eliminates the need for a high tension power source. However, the problem with this type of charger is that the charge distribution is apt to become irregular and, in addition, the charge potential is extremely susceptible to the environment. In fact, such a charger is far inferior to a charger of the type using a corona discharger in respect of the uniformity of charge distribution.
Actually, we found smears in the background of images produced in the early mornings of winter by a laser printer with a charge roller. This is presumably because the charge roller left at low temperature during the nighttime has the electric resistance thereof increased and, therefore, the charge potential of the photoconductive drum is lowered by about 200 V, compared to a normal temperature and humidity range, to thereby cause reversal development to occur.
Japanese Patent Laid-Open Publication No. 149668/1988 (referred to as Document 1 hereinafter) teaches that the uniformity of charge is noticeably improved when an AC voltage having a peak-to-peak voltage more than twice as high as a charge start voltage (V.sub.TH) is superposed in the event of application of a DC voltage. Japanese Patent Laid-Open Publication No. 132356/1981 (referred to as Document 2 hereinafter) discloses an implementation wherein a voltage is applied from a constant current power source to a charge roller in order to reduce the susceptibility of the charge roller to ambient conditions. Japanese Patent Laid-Open Publication No. 156476/1990 (referred to as Document 3 hereinafter) proposes an arrangement wherein an AC-superposed DC voltage is applied to a charge roller to insure stable charging with no regard to the environment. Further, Japanese patent Laid-Open publication No. 288174/1990 (referred to as Document 4 hereinafter) teaches means for heating a charge roller for the purpose of eliminating the dependency of the roller on the environment.
However, the scheme of Document 1 needs an AC power source in addition to a DC power source for superposing the AC voltage on the DC voltage, increasing the cost of the apparatus. Moreover, a great amount of AC current not contributing to the charge potential of the photoconductive element is wastefully consumed. This not only increases the running cost of the apparatus but also generates a great amount of ozone, bringing about the previously stated critical problems. With Document 2, it may be expected that due to the constant current power source the current and, therefore, the charge potential of the photoconductive element remains constant with no regard to the electric resistance of the charge roller. In practice, however, changes in leak current ascribable to the varying ambient conditions, particularly humidity, is not negligible. As a result, the current contributing to the charging of the photoconductive element changes to make the charge irregular in particular portions of the charged surface of the element. The approach of Document 3 increases the cost due to the AC power source although it reduces irregular charging. Further, the implementation of Document 4 increases the scale and cost of the apparatus since the heating means heats the entire charge roller.